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United States Patent |
5,019,473
|
Nguyen
,   et al.
|
May 28, 1991
|
Electrophotographic recording elements containing photoconductive
perylene pigments
Abstract
An electrophotographic recording element having a layer comprising a
photoconductive perylene pigment, as a charge generation material, that is
sufficiently finely and uniformly dispersed in a polymeric binder to
provide the element with excellent electrophotographic speed. The perylene
pigments are perylene-3,4,9,10-tetracarboxylic acid imide derivatives.
Inventors:
|
Nguyen; Khe C. (Pittsford, NY);
Gruenbaum; William T. (Rochester, NY)
|
Assignee:
|
Eastman Kodak Company (Rochester, NY)
|
Appl. No.:
|
485113 |
Filed:
|
February 23, 1990 |
Current U.S. Class: |
430/59.1; 430/73; 430/74 |
Intern'l Class: |
G03G 005/14 |
Field of Search: |
430/58,73,74
|
References Cited
U.S. Patent Documents
3752686 | Aug., 1973 | Kalz et al.
| |
4419427 | Dec., 1983 | Graser | 430/58.
|
4555467 | Nov., 1985 | Hasegawa et al.
| |
4578334 | Mar., 1986 | Borsenberger et al.
| |
4714666 | Dec., 1987 | Wiedemann et al.
| |
4792508 | Dec., 1988 | Kazmaier et al.
| |
Primary Examiner: Welsh; David
Attorney, Agent or Firm: Janci; David F.
Claims
What is claimed is:
1. An electrophotographic recording element having a layer comprising a
photoconductive perylene pigment that is sufficiently finely and uniformly
dispersed in a polymeric binder to provide the element with an
electrophotographic speed at least substantially equivalent to the
electrophotographic speed of an element having a corresponding layer
formed from the same pigment by vacuum sublimation in the absence of said
polymeric binder, said perylene pigment having the formula:
##STR93##
where each R is a phenethyl radical,
R.sup.1 is hydrogen, alkyl, cycloalkyl, aralkyl, aryl, heteroaryl, alkoxy,
mono- or dialkylamino, or when the compound of Formula I is a dimer,
R.sup.1 is 1,4-phenylene,
each Z is 2,3-naphthylene, 2,3-pyridylene, 3,4-pyridylene,
3,4,5,6-tetrahydro-1,2-phenylene, 9,10-phenanthrylene, 1,8-naphthylene,
the radical
##STR94##
where R.sup.2 is alkyl, cycloalkyl, aralkyl, aryl, heteroaryl, alkoxy,
dialkylamino, halogen, cyano, or nitro, or when the compound of Formula II
is a dimer, Z is 1,2,4,5-benzenetetrayl or 3,3',4,4'-biphenyltetrayl, and
m is a number from 0 to 4,
2. The electrophotographic recording element of claim 1, wherein the
perylene pigment has the formula I.
3. The electrophotographic recording element of claim 2, wherein R.sup.1 is
aralkyl.
4. The electrophotographic recording element of claim 2, R.sup.1 is
phenethyl.
5. The electrophotographic recording element of claim 2, wherein each of R
and R.sup.1 is phenethyl.
6. The electrophotographic recording element of claim 2, wherein R is
phenethyl and R.sup.1 is m-methyl-substituted phenethyl.
7. The electrophotographic recording element of claim 1, wherein the
perylene pigment has the formula II.
8. The electrophotographic recording element of claim 7, wherein Z is the
radical
##STR95##
9. The electrophotographic recording element of claim 8, wherein R is
phenethyl and m is 0.
10. The electrophotographic recording element of claim 1, wherein the
perylene pigment has the formula III.
11. The electrophotographic recording element of claim 1, wherein the
perylene pigment has a particle size up to about 0.2 micrometer.
12. The electrophotographic recording element of claim 1, wherein the
element is a multi-active element comprising a charge-generation layer
containing the photoconductive perylene pigment dispersed in a polymeric
binder, and a charge-transport layer.
Description
FIELD OF THE INVENTION
This invention relates to electrophotographic recording elements in general
and particularly to an electrophotographic element having a layer
containing a photoconductive perylene pigment dispersed in a polymeric
binder. More particularly, the invention relates to an electrophotographic
element containing a layer of finely-divided
perylene-3,4,9,10-tetracarboxylic acid imide pigment dispersed in a
polymeric binder. Such a layer exhibits unexpectedly good photosensitivity
and high resistance to abrasion, and is characterized by good durability.
BACKGROUND
In electrophotography an image comprising an electrostatic field pattern,
usually of non-uniform strength (also referred to as an electrostatic
latent image), is formed on an insulative surface of an
electrophotographic element comprising at least a photoconductive layer
and an electrically conductive substrate. The electrostatic latent image
is usually formed by imagewise radiation-induced dissipation of the
strength of portions of an electrostatic field of uniform strength
previously formed on the insulative surface. Typically, the electrostatic
latent image is then developed into a toner image by contacting the latent
image with an electrographic developer. If desired, the latent image can
be transferred to another surface before development.
In latent image formation the imagewise radiation-induced dissipation of
the initially uniform electrostatic field is brought about by the creation
of electron/hole pairs, which are generated by a material, often referred
to as a photoconductive or charge-generation material, in the
electrophotographic element in response to exposure to imagewise actinic
radiation. Depending upon the polarity of the initially uniform
electrostatic field and the types of materials included in the
electrophotographic element, part of the charge that has been generated,
i.e., either the holes or the electrons, migrates toward the charged
insulative surface of the element in the exposed areas and thereby causes
the imagewise dissipation of the initial field. What remains is a
non-uniform field constituting the electrostatic latent image.
Several types of electrophotographic recording elements are known for use
in electrophotography. In many conventional elements, the active
photoconductive or charge-generation materials are contained in a single
layer. This layer is coated on a suitable electrically conductive support
or on a non-conductive support that is overcoated with an electrically
conductive layer. In addition to single-active-layer electrophotographic
recording elements, various multi-active electrophotographic recording
elements are known. Such elements are sometimes called multi-layer or
multi-active-layer elements because they contain at least two active
layers that interact to form an electrostatic latent image.
A class of photoconductive materials useful in the aforementioned
single-active-layer and multiactive elements is the class of perylene
pigments, particularly perylene-3,4,9,10-tetracarboxylic acid imide
derivatives. Representative examples of patents pertaining to such
perylene photoconductive pigments include, U.S. Pat. No. 4,578,334, issued
Mar. 25, 1986, which describes multi-active electrophotographic recording
elements that contain, as photoconductive materials, certain crystalline
forms of N,N'-bis(2-phenethyl)perylene-3,4:9,10-bis(dicarboximide)
characterized by particular spectral absorption and x-ray diffraction
characteristics; U.S. Pat. No. 4,714,666, issued Dec. 22, 1987, which
describes single-active-layer electrophotographic elements and
multi-active elements containing, as photoconductive materials,
asymmetrically substituted perylene-3,4,9,10-tetracarboxylic acid imide
derivatives, and U.S. Pat. No. 4,792,508, issued Dec. 20, 1988, which
describes multi-active elements that contain as photoconductive materials,
mixtures of cis- and trans-naphthimidazole perylenes.
Unfortunately, electrophotographic recording elements of the prior art
which contain photoconductive perylene materials have typically suffered
from one or more disadvantages that have significantly restricted their
use. For example, vacuum sublimation (also known as vacuum deposition) is
frequently required to deposit photoconductive perylene pigments in a
crystal form suitable for high speed electrophotographic elements. Thus,
U.S. Pat. No. 4,578,334 describes a process wherein a perylene pigment is
deposited by vacuum sublimation in the form of an amorphous layer and is
thereafter converted to photoconductive crystalline form by contacting the
layer with an appropriate liquid composition. Vacuum sublimation, however,
is a batch process which makes production scale runs quite costly and thin
sublimed films are fragile and susceptible to damage until they can be
protected by a more durable overcoat.
To avoid the disadvantages inherent in forming photoconductive perylene
pigment layers using vacuum sublimation techniques and the fragile nature
of such layers; electrophotographic layers have been coated from liquid
coating compositions comprising finely-divided photoconductive perylene
pigments dispersed in solvent solutions of polymeric binders. See, for
example, U.S. Pat. No. 4,714,666. Electrophotographic layers coated from
such dispersions are more resistant to abrasion and more durable than the
layers formed by vacuum sublimation but, these advantages are obtained at
the expense of a considerable loss in electrophotographic speed. U.S. Pat.
No. 4,714,666 illustrates this point very well since such loss in speed is
evident from a comparison between electrophotographic speeds reported in
the working examples for electrophotographic recording elements containing
perylene pigments in dispersion coated layers and those elements
containing such pigments in vacuum deposited layers.
Also, dispersion coated layers containing photoconductive perylene pigments
provided by conventional prior art methods are deficient in several
respects, for example, the pigments have a relatively large particle size
and are poorly dispersed in the binder and do not form homogeneous layers
having the uniform distribution of fine particles that is necessary to
achieve optimum electrophotographic speed. In addition, such layers often
contain agglomerates of individual pigment particles and such agglomerates
detrimentally affect the image quality of copies formed with
electrophotographic elements containing the layers.
Conventional prior art procedures normally used for forming
dispersion-coated layers typically involve mixing the components of a
liquid coating composition, for example, a dispersion of photoconductive
perylene pigment in a solvent solution of polymeric binder, in a suitable
mixing device such as a ball mill or a paint shaker. As previously
indicated, such conventional procedures do not adequately disperse the
pigment particles and frequently form the aforementioned particle
agglomerates. Moreover, prolonged mixing of the photoconductive perylene
pigment in a device such as a ball mill can damage the pigment
structurally so that electrophotographic performance is detrimentally
affected.
It is an objective of this invention to provide electrophotographic
recording elements that comprise photoconductive perylene pigments and
have excellent photosensitivity, for example, photodischarge speed and
dark decay, but do not require vacuum sublimation techniques to achieve
such photosensitivity. It is also an objective of this invention to
provide electrophotographic recording elements comprising layers
containing photoconductive pigments dispersed in polymeric binders, which
layers are highly resistant to abrasion and exhibit good durability.
SUMMARY OF THE INVENTION
This invention provides an electrophotographic recording element that has a
layer in which photoconductive perylene pigment is dispersed in a
polymeric binder and exhibits excellent electrophotographic speed. Thus,
the electrophotographic recording element of this invention is an
electrophotographic recording element having a layer comprising a
photoconductive perylene pigment that is sufficiently finely and uniformly
dispersed in a polymeric binder to provide the element with an
electrophotographic speed at least substantially equivalent to the
electrophotographic speed of an element having a corresponding layer
formed from the same pigment by vacuum sublimation in the absence of said
polymeric binder. The perylene pigment has the formula:
##STR1##
where each R is a phenethyl radical,
R.sup.1 is hydrogen, alkyl, cycloalkyl, aralkyl, aryl, heteroaryl, alkoxy,
mono- or dialkylamino, or when the compound of Formula I is a dimer,
R.sup.1 is 1,4-phenylene,
each Z is 2,3-naphthylene, 2,3-pyridylene, 3,4-pyridylene,
3,4,5,6-tetrahydro-1,2-phenylene, 9,10-phenanthrylene, 1,8-naphthylene,
the radical
##STR2##
where R.sup.2 is alkyl, cycloalkyl, aralkyl, aryl, heteroaryl, alkoxy,
dialkylamino, halogen, cyano, or nitro, or when the compound of Formula II
is a dimer, Z is 1,2,4,5-benzenetetrayl or 3,3',4,4'-biphenyltetrayl, and
m is a number from 0 to 4,
The electrophotographic recording elements of this invention exhibit a
broad range of sensitivity, e.g., they exhibit electrophotographic
response over the visible region of the spectrum (400-700 nm), and in some
cases out into the infrared region, and often exhibit an unexpected
increase in electrophotographic response at all wavelengths within such
regions.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The photoconductive perylene pigments used in this invention are
3,4,9,10-tetracarboxylic acid imide derivatives that contain at least one
phenethyl radical and/or fused imidazo[1,2-a]pyridino ring moiety. Such
perylene pigments can be symmetrical or asymmetrical depending upon the
nature of the specific substituents, for example, the R.sup.1 or Z
radicals in formulas I, II or III. Also, while formula III specifically
sets forth the cis form of the perylene pigment, other forms such as trans
forms do exist and such forms of the pigments are included within the
scope of this invention.
The R radical in formula I or II is a phenethyl radical, i.e., a radical in
which an ethylene linkage joins a phenyl moiety to a 3,4-dicarboximide
nitrogen atom. The ethylene linkage and/or phenyl moiety can be
unsubstituted or can contain substituents that do not deleteriously affect
the photoconductive properties of the perylene pigment. Suitable
substituents of this type include for example, alkyl radicals, such as
methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl and
tert-butyl; cycloalkyl radicals such as cyclopropyl, cyclobutyl,
cyclopentyl and cyclohexyl; aralkyl radicals such as benzyl and phenethyl;
aryl radicals such as phenyl, chlorophenyl, anisyl, biphenyl and naphthyl;
heteroaryl radicals such as pyridyl, pyrimidyl, thiophenyl, pyrrolyl and
furyl; alkoxy radicals such as methoxy and ethoxy; dialkylamino radicals
containing the same or different alkyls such as dimethylamino,
diethylamino, and methylbenzylamino; and halogen such as chlorine, bromine
or fluorine. In addition to the specific R.sup.1 radicals set forth in
formula I, illustrative R.sup.1 substituents include alkyl radicals such
as methyl, ethyl, propyl, isopropyl, butyl, pentyl, hexyl, methoxyethyl
and methoxypropyl; cycloalkyl radicals such as cyclopropyl, cyclobutyl,
cyclopentyl and cyclohexyl; aralkyl radicals such as benzyl, phenethyl,
phenylpropyl and phenylbutyl; aryl radicals such as phenyl, tolyl, xylyl,
biphenylyl and naphthyl; and heteroaryl radicals such as pyridyl and
pyrimidyl.
Some illustrative R.sup.2 substituents in formulas II and III include alkyl
radicals, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl,
sec-butyl, and tert-butyl; cycloalkyl radicals such as cyclopropyl,
cyclobutyl, cyclopentyl and cyclohexyl; aralkyl radicals such as benzyl
and phenethyl; aryl radicals such as phenyl, chlorophenyl, anisyl,
biphenyl and naphthyl; heteroaryl radicals such as pyridyl, pyrimidyl,
thiophenyl, pyrrolyl and furyl; alkoxy radicals such as methoxy and
ethoxy; dialkylamino radicals containing the same or different alkyls such
as dimethylamino, diethylamino, and methylbenzylamino; and halogen such as
chlorine, bromine or fluorine.
Although the R, R.sup.1 and R.sup.2 radicals generally contain only carbon
and hydrogen, they often contain additional atoms such as oxygen,
nitrogen, sulfur and halogen. Also, it is evident from the previous
description of formula II and III and the following Tables 2 and 3 that
the imidazo[1,2-a]-pyridino ring moiety (which includes the Z substituent)
in the photoconductive perylene pigments employed in the practice of this
invention can contain a wide variety of substituents, including fused ring
systems of carbon or carbon and hetero atoms, each ring containing 5 or
more carbon or carbon and hetero atoms such as fused benzene, naphthalene,
pyrimidine or pyridine rings.
Symmetrical perylene 3,4,9,10-tetracarboxylic acid imide derivatives that
can be used in the practice of this invention are conveniently prepared by
cyclizing perylene tetracarboxylic dianhydrides with an excess of suitable
organic amines such as phenylethyl amine or diaminonaphthalene. Typical
procedures are described in U.S. Pat. No. 4,156,757, issued May 29, 1979,
and in U.S. Pat. Nos. 4,578,334; and 4,792,508, referred to previously
herein. Typical procedures for preparing asymmetrical
perylene-3,4,9,10-tetracarboxylic acid imide derivatives employed in the
practice of this invention are described in U.S. Pat. No. 4,714,666,
previously referred to herein. Synthesis of dimeric
phenylene-3,4,9,10-tetracarboxylic acid imide derivatives can be carried
out by methods analogous to those described in U.S. Pat. No. 4,714,666
except that at least 2 moles of a perylene tetracarboxylic acid
monoanhydride monoimide is cyclized by reaction with 1 mole of an
appropriate polyfunctional organic amine such as 1,4-phenylenediamine or
1,2,4,5-benzenetetraamine.
A partial listing of perylene pigments of formula I that can be used in
this invention is set forth in the following Table 1 where R and R.sup.1
in that formula I are set forth.
TABLE 1
__________________________________________________________________________
##STR3## (I)
Pigment
R R.sup.1
__________________________________________________________________________
P-1
##STR4##
##STR5##
P-2
##STR6##
##STR7##
P-3
##STR8##
##STR9##
P-4
##STR10## CH.sub.2 CH.sub.2 CH.sub.3
P-5
##STR11##
##STR12##
P-6
##STR13## CH.sub.2 CH.sub.2 CH.sub.2 OCH.sub.3
P-7
##STR14## H
P-8
##STR15##
##STR16##
P-9
##STR17## CH.sub.2 CH.sub.2 OCH.sub.3
P-10
##STR18## CH.sub.2 CH.sub.2 CH.sub.2 SCH.sub.3
P-11
##STR19##
##STR20##
P-12
##STR21##
##STR22##
P-13
##STR23##
##STR24##
P-14
##STR25##
##STR26##
P-15
##STR27##
##STR28##
P-16
##STR29##
##STR30##
P-17
##STR31##
##STR32##
P-18
##STR33## CH.sub.3
P-19
##STR34##
##STR35##
P-20
##STR36##
##STR37##
P-21
##STR38##
##STR39##
P-22
##STR40##
##STR41##
P-23
##STR42## CH.sub.2 CH.sub.2 CH.sub.2 OCH.sub.3
P-24
##STR43## CH.sub.2 CH.sub.2 CH.sub.2 OCH.sub.3
P-25
##STR44##
##STR45##
P-26
##STR46##
##STR47##
P-27
##STR48##
##STR49##
P-28
##STR50##
##STR51##
P-29
##STR52##
##STR53##
P-30
##STR54##
##STR55##
P-31
##STR56##
##STR57##
P-32
##STR58##
##STR59##
P-33
##STR60##
##STR61##
P-34
##STR62##
##STR63##
P-35
##STR64##
##STR65##
P-36
##STR66##
##STR67##
P-37
##STR68##
##STR69##
__________________________________________________________________________
A partial listing of perylene pigments of formula II that can be used in
this invention is set forth in the following Table 2. In each case R in
formula II is phenethyl and Z, R.sup.2 and m are as defined in the Table.
TABLE 2
__________________________________________________________________________
##STR70## (II)
Pigment Z R.sup.2 m
__________________________________________________________________________
P-38
##STR71## -- --
P-39
##STR72## CH.sub.3
1
P-40
##STR73## Cl 1
P-41
##STR74## NO.sub.2
1
P-42
##STR75## F 1
P-43
##STR76## -- --
P-44
##STR77## -- --
P-45
##STR78## -- --
P-45a
##STR79## -- --
P-46
##STR80## -- --
P-47
##STR81## -- --
*P-48
##STR82## -- --
*P-50
##STR83## -- --
__________________________________________________________________________
*Dimers
A partial listing of perylene pigments of formula III that can be used in
this invention is set forth in the following Table 3 where each Z is the
same and, R.sup.2 and m in formula III are as defined.
TABLE 3
__________________________________________________________________________
##STR84## (III)
Pigment Z R.sup.2 m
__________________________________________________________________________
P-51
##STR85## -- --
P-52
##STR86## -- --
P-53
##STR87## -- --
P-54
##STR88## -- --
P-55
##STR89## Cl 1
P-56
##STR90## NO.sub.2
1
P-57
##STR91## -- --
P-58
##STR92## -- --
__________________________________________________________________________
The photoconductive layers in the electrophotographic recording elements of
this invention are prepared using unique coating compositions in which
finely-divided perylene pigments are very uniformly dispersed in a solvent
solution of polymeric binder. Briefly, such coating compositions are
prepared by a method that comprises the steps of (1) milling a crude
perylene pigment having a formula I, II or III with milling media
comprising inorganic salt and non-conducting particles under shear
conditions in the substantial absence of binder solvent to provide pigment
having a particle size up to 0.2 micrometer, (2) continuing the milling at
higher shear and a temperature up to about 50.degree. C., to achieve a
perceptible color change of the pigment particles, (3) rapidly reducing
the temperature of the milled pigment by at least 10.degree. C., (4)
separating the milled pigment from the media and (5) mixing the milled
pigment with a solvent solution of polymeric binder to form the coating
composition. A very high degree of dispersion of photoconductive perylene
pigment in solvent solution of polymeric binder is achieved by this
method. This is quite unexpected since it has been our experience that
3,4,9,10-tetracarboxylic acid imide derivatives of the type having
formulas I, II and III are particularly difficult to effectively disperse
in liquid coating compositions used to form electrophotographic layers.
The crude perylene pigment used to form the coating composition is an
as-synthesized pigment and has a much larger particle size than does the
electrophotographic quality pigment, i.e., the photoconductive perylene
pigment. Also, perylene pigments are known to exhibit polymorphism, i.e.,
they are capable of existing in various crystal forms, as well as
amorphous forms. The milling method provides a perylene pigment that is in
a finely-divided photoconductive form capable of achieving a high degree
of dispersion in electrophotographic coating compositions. While the exact
mechanism whereby the process functions to achieve the improved results is
not known with certainty, during milling the solvent and polymeric binder
are not brought into association with the pigment particles until such
particles are finely-divided and free from agglomerates. Accordingly, any
adverse influences due to the presence of polymeric binder and/or solvent
on the formation of finely-divided particles and breaking up of
agglomerates and dispersion of individual particles are avoided. After
milling, the particles can be effectively dispersed in the solvent
solution of polymeric binder using a conventional mixing device such as a
media mill or a paint shaker to form the coating composition. Such pigment
particles have a very uniform size distribution and the size of the
individual particles does not exceed 0.2 micrometer. While the milling
process can be applied to mixtures of two or more perylene pigments,
optimum electrophotographic properties are generally obtained by milling
the pigments separately and then adding them to the coating composition
which is subjected to conventional mixing techniques prior to dispersion
coating the electrphotographic recording element.
During the first milling stage, the perylene pigment is mechanically ground
in the dry state under shear conditions that break up particle
agglomerates and provide particles having a very small size. As
synthesized, perylene pigments normally have a particle size that is too
large for them to be effectively used in electrophotographic applications.
In this condition, they are known in the prior art as "crude" pigments.
Such crude pigments normally have a particle size in excess of 10
micrometers, often a particle size of 50 micrometers, and in at least some
cases, at least 1 millimeter. In this first milling stage, the particle
size is reduced to a particle size that does not exceed about 0.2
micrometer, typically a particle size of about 0.02 to 0.2 micrometer and
often about 0.05 to 0.1 micrometer. The pigment particles have a variety
of shapes, e.g., elongated, needle-like, spherical, regular or irregular.
The practical size referred to herein is the largest dimension of the
particle and can be readily determined from electron photomicrographs
using techniques well known to those skilled in the art. Milling is
carried out in the substantial absence of the solvent and the polymeric
binder, i.e., there is either none of these ingredients present or, if
some polymeric binder and/or solvent is included, it is in an amount so
small as to have no significant detrimental effect on the pigment
particles.
In the first stage of the milling, the perylene pigment particles are
milled under shear such that the particle size of the pigment is reduced
to at least 0.2 micrometer and the pigment and milling media form a
homogeneous mixture. Milling apparatus capable of providing such shear
with the milling mixture are well known and include, e.g., conventional
ball mills, roll mills, paint shakers, vibrating mills and the like.
Examples of milling apparatus that can utilize shearing are described in
U.S. Pat. Nos. 4,555,467, issued Nov. 26, 1985 and 3,752,686, issued Aug.
14, 1973. The shear employed with a given mixture is subject to variation,
as is obvious to those skilled in the art, depending upon such things as
the type of milling apparatus, milling media and perylene pigment
selected. However, the energy applied to the non-conductive particles in
the milling media which results in appropriate shear in the first milling
stage generally does not exceed about 5 watts, and is typically in the
range of about 3 to 5 watts.
The milling media used to grind the perylene pigment comprises two
components, i.e., inorganic salt particles and non-conducting particles in
a weight ratio of about 0.5:1 to 3:1, typically about 1:1 to 2:1. Examples
of inorganic salts include alkali metal halides, carbonates, sulfates or
phosphates such as sodium chloride, potassium bromide, sodium sulfate,
potassium sulfate, sodium carbonate, and sodium phosphate. In prior art
milling methods where such inorganic salt particles are used in milling
media with other particles, e.g., steel balls, they are normally used as
milling aids at considerably lower concentrations. Such salts are
typically separated from the milled pigment by washing with water since
they often have a high degree of solubility in water, e.g., a solubility
of at least 200 and often 400 grams of salt per liter of water. Examples
of non-conductive particles include materials such as glass particles,
zirconium oxide particles and organic polymeric beads such as polymethyl
methacrylate beads that are electrically non-conducting. Non-conductive
particles are employed because they do not acquire charges due to
triboelectrification which charges would cause pigment to adhere to the
particles. Furthermore, the use of non-conducting particles limits
corrosion due to the presence of the inorganic salt particles that might
otherwise occur under the milling conditions. The inorganic salts
typically have particle sizes in the range of about 5 to 500 micrometers
while the particle size of the non-conducting particles is normally in the
range of about 0.5 mm to about 5 mm.
Following comminution of the crude pigment in the first milling stage,
milling is continued in a second stage at higher shear and at a
temperature up to 50.degree. C. Milling is continued until there is a
perceptible color change of the pigment. This is the point at which there
is a just noticeable difference in the color of the pigment which can be
detected by observation with the unaided human eye. It is also interesting
to note that the perylene pigment is substantially completely adsorbed to
the surfaces of the inorganic salt particles when milling is completed.
This is an excellent indicator of milling completion. During this second
milling stage, shear can be increased simply by increasing the
concentration of milling media. However, it is often convenient to simply
transfer the milled composition from the first stage milling (comprising
pigment and milling media) to a device that will develop increased shear
relative to the shear used in the first stage. For example, where a ball
mill is used is the first stage, this can be followed by using an attritor
in the second milling stage, as illustrated in the following Examples.
However, other devices such as jet mills or high speed roll mills are
suitable for use for the second milling stage. The milling temperature in
the second stage does not exceed about 50.degree. C. and is generally in
the range of about 0.degree. C. to 50.degree. C., typically in the range
of about 20.degree. C. to about 45.degree. C. The milling time, in stages
1 and 2 will vary greatly, depending upon a number of factors such as the
relative proportions of pigment and milling media and the specific milling
equipment utilized. Generally, a suitable time for the stage 1 milling can
be as much as 240 hours with typical times being in the range of about 72
hrs to 120 hrs, while, in the second stage, the milling time is generally
about 10 min to 5 hrs, often about 30 min to 90 min. Typically, the
concentration of the perylene pigment during milling is about 0.01% to
10%, often about 0.5% to 5% by weight, based on the weight of milling
media. The milling operation tends to result in a liberation of heat which
raises the temperature of the milling composition, i.e., the mixture of
pigment and milling media. The milling apparatus is, therefore, normally
equipped with cooling means to keep the temperature below 50.degree. C.
Upon completion of stage 2 milling, the temperature of the milled pigment
is rapidly reduced by at least 10.degree. C. often by 10.degree. C. to
60.degree. C. The rapid reduction in temperature stabilizes the pigment
against changes in morphology and crystal form prior to its addition to
the solvent solution of polymeric binder. It is usually convenient to
reduce the temperature of the milled mixture by quenching with water, for
example, ice water or room temperature water depending upon the
temperature of the milled mixture. However, other cooling means, for
example, ice or cold air, can be used, but water is preferred since it
dissolves the inorganic salt particles which facilitates recovery of the
pigment. The non-conducting solid particles can be removed from the
mixture using any suitable means such as filtration or centrifuging.
Following separation of the milled pigment from the milling media, the
pigment is mixed with a solvent solution of polymeric binder to form an
electrophotographic coating composition. The pigment can be mixed with the
solvent solution of polymeric binder immediately or it can be stored for
some period of time before making up the coating composition. The
polymeric binder used in the preparation of the coating composition can be
any of the many different binders that are useful in the preparation of
electrophotographic layers. Representative materials that can be employed
as binders in the practice of this invention are film-forming polymers
having a fairly high dielectric strength and good electrically insulating
properties. Such binders include, for example, styrene-butadiene
copolymers; vinyl toluene-styrene copolymers; styrene-alkyd resins;
silicone-alkyd resins; soya-alkyd resins; vinylidene chloride-vinyl
chloride copolymers; poly(vinylidene chloride); vinylidene
chloride-acrylonitrile copolymers; vinyl acetate-vinyl chloride
copolymers; poly(vinyl acetals), such as poly(vinyl butyral); nitrated
polystyrene; poly(methylstyrene); isobutylene polymers; polyesters, such
as poly[ethylene-co-alkylenebis(alkyleneoxyaryl)-phenylenedicarboxylate];
phenolformaldehyde resins; ketone resins; polyamides; polycarbonates;
polythiocarbonates;
poly[ethylene-co-isopropylidene-2,2-bis(ethyleneoxyphenylene)terephthalate
]; copolymers of vinyl haloacrylates and vinyl acetate such as
poly(vinyl-m-bromobenzoate-co-vinyl acetate); chlorinated poly(olefins),
such as chlorinated poly(ethylene); cellulose derivatives such as
cellulose acetate, cellulose acetate butyrate and ethyl cellulose; and
polyimides, such as poly[1,1,3-trimethyl-3-(4'-phenyl)-5-indane
pyromellitimide].
Suitable organic solvents for forming the polymeric binder solution can be
selected from a wide variety of organic solvents, including, for example,
aromatic hydrocarbons such as benzene, toluene, xylene and mesitylene;
ketones such as acetone, butanone and 4-methyl-2-pentanone; halogenated
hydrocarbons such as methylene chloride, chloroform and ethylene chloride;
ethers, including ethyl ether and cyclic ethers such as dioxane and
tetrahydrofuran; and mixtures thereof. The amount of solvent used in
forming the binder solution is typically in the range of from about 2 to
about 100 parts of solvent per part of binder by weight, and preferably in
the range of from about 10 to about 50 parts of solvent per part of binder
by weight.
As previously indicated herein, the electrophotographic elements of this
invention can be of various types, all of which contain photoconductive
perylene derivative that serve as charge-generating materials in the
elements. Such elements include both those commonly referred to as single
layer or single-active-layer elements and those commonly referred to as
multiactive, multilayer, or multi-active-layer elements which have been
briefly referred to previously herein. All of these elements exhibit an
electrophotographic speed that is at least equal to comparable
electrophotographic recording elements in which the photoconductive
perylene derivative is vacuum deposited.
Single layer elements contain one layer that is active both to generate and
to transport charges in response to exposure to actinic radiation. Such
elements typically comprise at least an electrically conductive layer in
electrical contact with a photoconductive layer. In single layer elements
prepared as described herein, the photoconductive layer contains at least
one photoconductive perylene pigment as the charge-generation material to
generate charge in response to actinic radiation and a transport material
which is capable of accepting charges generated by the charge-generation
material and transporting the charges through the layer to effect
discharge of the initially uniform electrostatic potential. The
photoconductive layer is electrically insulative, except when exposed to
actinic radiation, and contains an electrically insulative film-forming
polymeric binder.
Multiactive elements contain at least two active layers, at least one of
which is capable of generating charge in response to exposure to actinic
radiation and is referred to as a charge-generation layer (hereinafter
also referred to as a CGL), and at least one of which is capable of
accepting and transporting charges generated by the charge-generation
layer and is referred to as a charge-transport layer (hereinafter also
referred to as a CTL). Such elements typically comprise at least an
electrically conductive layer, a CGL, and a CTL. Either the CGL or the CTL
is in electrical contact with both the electrically conductive layer and
the remaining CGL or CTL. The CGL contains at least a photoconductive
material that serves as a charge-generation material; the CTL contains at
least a charge-transport material; and either or both layers can contain
an additional film-forming polymeric binder. In multiactive elements of
this invention the charge-generation material is at least one
photoconductive perylene pigment dispersed in a polymeric binder and the
element contains a CTL. Any suitable charge-transport material can be used
in such CTL's.
Single layer and multilayer electrophotographic elements and their
preparation and use, in general, are well known and are described in more
detail, for example, in U.S. Pat. Nos. 4,701,396; 4,714,666; 4,666,802;
4,578,334; 4,175,960; 4,514,481; and 3,615,414, the disclosures of which
are hereby incorporated herein by reference.
In preparing single-active-layer electrophotographic elements of the
invention, the components of the photoconductive layer, including any
desired addenda, can be dissolved or dispersed in the coating composition
and then coated on an electrically conductive layer or support. The
solvent for the polymeric binder is then allowed or caused to evaporated
from the mixture to form the permanent layer containing from about 0.01 to
50 weight percent of the charge-generation material and about 10 to 70
weight percent of a suitable charge transport material.
In preparing multiactive electrophotographic elements, the components of
the CTL can similarly be dissolved or dispersed in the coating composition
and can be coated on either an electrically conductive layer or support or
on a CGL previously similarly coated or otherwise formed on the conductive
layer or support. In the former case a CGL is thereafter coated on the
CTL.
Various electrically conductive layers or supports can be employed in the
electrophotographic recording elements of this invention, such as, for
example, paper (at a relative humidity above 20 percent); aluminum-paper
laminates; metal foils such as aluminum foil and zinc foil; metal plates
such as aluminum, copper, zinc, brass and galvanized plates; vapor
deposited metal layers such as silver, chromium, vanadium, gold, nickel,
and aluminum; and semiconductive layers such as cuprous iodide and indium
tin oxide. The metal or semiconductive layers can be coated on paper or
conventional photographic film bases such as poly(ethylene terephthalate),
cellulose acetate and polystyrene. Such conducting materials as chromium
and nickel can be vacuum-deposited on transparent film supports in
sufficiently thin layers to allow electrophotographic elements prepared
therewith to be exposed from either side.
When a photoconductive layer of a single-active-layer element or a CGL of a
multiactive element is dispersion coated as described herein, the
polymeric binder may, if it is electrically insulating, help to provide
the element with electrically insulating characteristics. It also is
useful in coating the layer, in adhering the layer to an adjacent layer,
and when it is a top layer, in providing a smooth, easy to clean,
wear-resistant surface. A significant feature of this invention is that an
electrophotographic recording element of this invention which contains a
CGL formed as described herein contains a photoconductive perylene pigment
in a polymeric binder and, therefore exhibits a surface that is much more
durable than a comparable layer containing the same perylene pigment but
formed by vacuum sublimation. This is advantageous in manufacturing
operations where such a CGL is subjected to handling prior to overcoating
with, for example, a CTL.
The optimum ratio of charge-generation material (perylene pigment) to
polymeric binder may vary widely depending upon the particular material
employed. The charge generation material can be a single pigment or it can
be two or more pigments. In general, useful results are obtained when the
amount of active charge-generation material contained within the layer is
within the range of from about 0.01 to 90 weight percent, based on the dry
weight of the layer.
Electrophotographic recording elements of this invention can optionally
contain other addenda such as leveling agents, surfactants, plasticizers,
sensitizers, contrast-control agents, and release agents and they can be
coated using any of the wide variety of suitable coating techniques known
in the art for forming such elements such as, for example, knife coating,
gravure coating or hopper coating. Also, such elements can contain any of
the optional additional layers known to be useful in electrophotographic
recording elements in general, such as, e.g., subbing layers, overcoat
layers, barrier layers, and screening layers.
Electrophotographic recording elements having vacuum deposited charge
generation layers comprising a photoconductive perylene pigment are well
known in the art, as illustrated for example, by U.S. Pat. Nos. 4,578,334,
4,714,666, and 4,792,508, referred to previously herein. Generally the
charge generation layer is first formed as a substantially amorphous layer
of photoconductive perylene pigment by vacuum sublimation. Vacuum
sublimation is conveniently effected by placing the photoconductive
perylene pigment in a crucible contained in a vacuum deposition apparatus
and positioning the substrate relative to the crucible so that materials
subliming from the crucible will be deposited upon the substrate. The
vacuum chamber is typically maintained at a pressure of from about
5.times.10.sup.-4 to about 5.times.10.sup.-5 Torr, depending upon such
variables as the pigment or substrate used. The crucible is heated to a
minimum temperature consistent with an adequate rate of sublimation of the
perylene pigment. Temperatures in the range of 250.degree. C. to about
400.degree. C. are typical. To facilitate formation of an amorphous layer,
the substrate is maintained at a temperature at or below room temperature.
Following vacuum deposition, the amorphous perylene layer is converted to a
crystalline photoconductive form having increased photosensitivity. This
is normally accomplished by exposing the pigment layer to solvent vapor or
treating the pigment layer with a liquid solvent. The latter technique is
preferred when it is desired to coat a charge transport layer over the
vacuum deposited perylene pigment layer.
By appropriate manipulation of conditions and choice of specific materials
known to those skilled in the art an electrophotographic recording element
comprising a vacuum deposited perylene pigment layer having a composition
and thickness for appropriate comparison with an electrophotographic
recording element of this invention can be readily obtained without undue
experimentation. The same materials are used to form the
electrophotographic recording element by vacuum deposition or by
dispersion coating and include, for example, the same perylene pigments,
charge transfer materials and supports. To provide comparable
electrophotographic recording elements it may be necessary to adjust the
thickness of various layers, for example, a charge generation layer and/or
a charge transport layer of a specific element such as a multi-active
layer electrophotographic recording element. However, such variables are
well known to those skilled in the art and do not form a part of this
invention. The significant point is that the electrophotographic recording
elements of this invention exhibit an electrophotographic speed that is at
least equal to that of a comparable electrophotographic recording element
in which the perylene pigment layer is formed by vacuum sublimation.
The following examples are presented to further illustrate the invention.
For convenience, the perylene pigments are identified in such examples by
the "P" number corresponding to that pigment in Table 1, 2 or 3, as
previously described.
EXAMPLE 1
A ball mill of 3750 cc capacity was charged with 1800 g of glass beads with
a diameter of 2 mm and 1800 g of sodium chloride particles having a
diameter of 500 micrometers and 180 g of gold P-38 pigment having a
particle size of 0.5 mm. The mixture was then sheared by milling for 72
hrs. at a temperature of 25.degree. C. The resulting mixture was
homogeneous and contained gold P-38 pigment that had a particle size of
0.2 micrometer.
The milled mixture obtained from the first stage was transferred to an
attritor dry grinding vessel having 10 liters capacity and containing a
stirrer having a rotating shaft containing 2 pairs of arms fixed to the
rotating shaft and extending toward the side wall of the vessel. 2500 g
more of the glass beads and 2000 g more of the sodium chloride particles
were added to the attritor and the mixture was agitated at 500 rpm for 70
minutes at a temperature of 45.degree. C. These conditions increased the
shear on the mixture in comparison to the first stage. The P-38 pigment
changed from gold to a bright pink/purple color and was adhered to the
surface of the inorganic salt particles. The glass beads were removed from
the mixture and the pigment and salt particles were stirred in ice for 2
hours. The resulting pigment-sodium chloride mixture was stored at
0.degree. C. for approximately 48 hours, washed free of sodium chloride
with distilled water and dried at room temperature. The separated P-38
pigment was bright pink/purple, had a particle size of 0.1 micrometer and
exhibited peaks at diffraction angles (2.THETA.) of 5.59.degree.,
9.85.degree., 11.5.degree., 25.2.degree. and 25.9.degree.. In comparison,
the crude pigment exhibited a more crystalline diffraction pattern with
diffraction peaks at 5.96.degree., 10.0.degree., and 12.9.degree. in the
X-ray diffraction pattern obtained with CuK.alpha. radiation.
A coating composition for forming a charge-generation layer (CGL) was
prepared by adding 3 g of the P-38 pigment particles and 1 g of
polyvinylbutyral binder to 96 g of methylisobutyl ketone and ball milling
for 72 hours. The composition was diluted to 4.5 percent solids with
methylisobutyl ketone. The resulting dispersion was coated on a conductive
support comprising a thin conductive layer of nickel on poly(ethylene
terephthalate) film to provide a CGL of 0.7 micrometer thickness.
A coating composition for forming a charge-transport layer (CTL) was
prepared comprising 11 weight percent solids dissolved in dichloromethane.
The solids comprised 4 g of
1,1-bis(4-di-p-tolylaminophenyl)-3-phenylpropane, a charge-transport
material, and 6 g of a binder comprising bisphenol A polycarbonate. The
coating composition was coated onto the CGL and dried to a thickness of 18
micrometers. The resulting multi-active layer electrophotographic
recording element was then charged to a uniform potential of -500 V,
exposed at its maximum absorption wavelength of 520 nm and discharged to
-100 V. The energy required in ergs/cm.sup.2 (photodecay) was 5.8
ergs/cm.sup.2. The dark discharge rate for the element (dark decay)
observed 10 seconds after charging was 4 V/sec.
For comparison, a multi-active layer electrophotographic recording element
was prepared by vacuum deposition using the materials described previously
in this Example 1. In preparing the element, a 0.25 micrometer thick layer
of P-38 was vacuum deposited on the support by sublimation from a
resistance-heated tatalum crucible at a temperature of 350.degree. C., a
pressure of 5.times.10.sup.-5 Torr, and a crucible to support distance of
15 cm. The nickel coated poly(ethylene terephthalate) support was at a
temperature of 50.degree. C. The vacuum deposited layer was overcoated at
a temperature of 25.degree. C. with the CTL coating composition described
previously in this Example 1 and dried to give a thickness of 18
micrometers. The resulting multi-active layer electrophotographic
recording element was then charged and exposed under the same conditions
as the dispersion coated element prepared previously in this Example 1.
The photodecay was 11 ergs/cm.sup.2 and the dark decay was 2 V/sec. Thus
the electrophotographic recording element of this invention exhibited a
two-fold increase in electrophotographic speed compared to the
electrophotographic recording element prepared using vacuum sublimation.
EXAMPLE 2
The procedure of Example 1 was repeated except that the P-38 perylene
pigment was replaced with different perylene pigments. The pigments used
and the photodecay values obtained with the electrophotographic recording
elements prepared and tested according to the procedures of Example 1, are
reported in the following Table.
TABLE
______________________________________
Vacuum Deposited
Electrophotographic
Element of Invention
Recording Element
Perylene
Photodecay
Dark Decay Photodecay
Dark Decay
Pigment
(ergs/cm) (V/sec) (ergs/cm)
(V/sec)
______________________________________
P-1 3.1 2.5
P-3 2.6 2.9
P-39 8 10
P-40 15 16
P-44 9 14
P-45 20 30
P-51 15 22
P-54 15 22
______________________________________
The photodecay values reported in the above table clearly demonstrate that
the electrophotographic recording elements of this invention meet or
exceed the electrophotographic speed for comparable electrophotographic
recording elements containing vacuum deposited photoconductive perylene
pigments.
The invention has been described in detail with particular reference to
preferred embodiments thereof, but it will be understood that variations
and modifications can be effected within the spirit and scope of the
invention.
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